In order to cope with the increasing demand for subscriber services on telecommunication networks, digital optical transmission networks are being introduced in which local access networks, which typically route electrical signals, are interconnected by a fibre optic network. This has been facilitated by the introduction of the Synchronous Digital Hierarchy (SDH) standard that deals with the formatting of signals in Synchronous Transfer Modules (STM) which will allow a unified telecommunication infrastructure and provides improved flexibility by permitting electrical digital signals from the local access networks to be converted into optical signals and transmitted through the optical network.
In order to upgrade the capacity of the optical network, Wavelength-Division-Multiplexing (WDM) techniques have been proposed, which permit the transmission capacity of a fibre link to be upgraded to the multi-Gbit/s range.
Thus, it has been proposed to provide a multi-Gbit/s WDM network superimposed on the top level of a SDH network. For a fuller discussion, reference is directed to I. Hawker, "Evolution of Digital Optical Transmission Networks", BT Technol. J., Vol. 9, No. 4, October 1991, pp 43-5. It is proposed to include nodes in the WDM network to allow dynamic re-routing of WDM data streams. It is proposed that each node should include access ports to allow optical streams to be dropped to and added from lower levels of the network. It has been proposed that routing of the high bit rate streams is directly performed in the optical layer whereas processing of any of the streams is achieved in the electronic domain by dropping it to a lower layer of the network. Network management and administration is proposed to be achieved from a management unit which configures an optical cross-connect at each node according to traffic requirements. Thus, optical cross-connects (OXC's) constitute the routing nodes of the WDM transport network.
A number of prior proposals have been made for the node architecture of the OXC and an example is given in FIG. 1.
Three input fibres 1, 2, 3 are connected at the node to output fibres 4, 5, 6. Each of the fibres 1-3 carries four WDM wavelength channels, which are connected to the output fibres 4-6 by means of space switches X.sub.1 -X.sub.4. In this example, the space switches are 4.times.4 matrices which thus have four inputs a-d and four outputs e-h that are optically interconnectable and are controlled by applied electrical signals (not shown). Such space switches are well known and include electrically controllable elements of variable refractive index to produce selective switching between the inputs and outputs.
The four optical channels of each input fibre 1-3 are applied to an input of each of the space switches X.sub.1 -X.sub.4 and each is separately selected by means of a respective tunable filter F1.sub.1, F2.sub.1, F3.sub.1, F4.sub.1. Similarly, the various wavelength channels from fibres 2 and 3 are applied to respective inputs of the space switches through filters F1.sub.2 -F4.sub.2, F1.sub.3 -F4.sub.3. The fourth input of each space switch X.sub.1 -X.sub.4 is connected to tunable optical transmitter units 7 which produce optical signals of a single frequency in response to digital electrical signal streams from the lower electrical layer of the network, to allow electrical signals to be added into the optical network for transmission in the optical layer. The electrical signals may be SDH, PDH, ATM, X25 or any other suitable format, since the optical network can transmit the bit stream transparently in the optical domain. Similarly the outputs h of each of the space switches X.sub.1 -X.sub.4 are connected to receiver units 8 that include photodetectors for producing electrical signals, so that digital data streams can be dropped out of the optical network into the lower, electrical layer. The outputs e-g of the space switches X.sub.1 -X.sub.4 are respectively connected to the inputs to the fibres 4, 5 and 6 by means of combiners 9, 10, 11. Thus, by applying control signals to the space switches X.sub.1 -X.sub.4 traffic in the WDM channels of input fibres 1-3 can be switched selectively between output fibres 4-6 and signal trains can be added to and dropped out of the optical network by means of the transmitters and receivers 7, 8. This arrangement however suffers from a number of disadvantages.
Firstly, the ultimate size of the OXC is limited by the size of the space switches. Currently, these are made in LiNbO.sub.3 or InP and are limited for practical purposes to 8.times.8. Also, for polarisation independent operation they typically require high switching voltages of the order of +/-100 V and have long rise times &gt;100 ns. Furthermore, the devices are not at present hermetically sealed, and the material thereof tends to exhibit drift characteristics, so as to alter the performance with time.
Secondly, a node configuration as shown in FIG. 1 produces a loss of approximately 30 dB and the main contributor to the loss is the switch array. In order to overcome this problem, optical fibre amplifiers such as erbium doped fibre amplifiers need to be used to provide amplification.
Thirdly, the node architecture exhibits blocking characteristics. It is not possible for the same wavelength channel from two of the input fibres to leave on the same output fibre.